1. Field of the Invention
This invention relates to the esterification of phenolic compounds using a catalyst consisting essentially of an alkali metal compound and a boron-containing compound. More particularly, this invention relates to the production of diphenyl esters of aromatic nonvicinal benzene dicarboxylic acids using a catalyst consisting essentially of an alkali metal compound and a boron compound wherein said alkali metal compound is at least one member selected from the group consisting of alkali metal salts of boric acid, alkali metal polyborates, alkali metal hydroxides, and alkali metal borohydrides and said boron compound is at least one member selected from the group consisting of boric acid, boric anhydride, alkali metal polyborates, alkali metal salts of boric acid and alkali metal borohydrides.
Lowrance in U.S. Pat. No. 3,772,389 discloses that prior to his invention it was impossible to form phenyl esters directly from the corresponding phenolic compound and carboxylic acids and that the synthesis required the use of the corresponding carboxylic acid chloride. Lowrance discloses that the direct esterification can be carried out using a catalytic amount of a borate-sulfuric acid complex at a suitable temperature and for a suitable time. Examples 1 and 2 of Lowrance indicate that esterification is impossible using sulfuric acid alone or boric acid alone as the catalyst, while Examples 4-7 illustrate that the boric acid sulfuric acid catalyst system cannot be replaced with zinc chloride, p-toluene sulfonic acid, boron trifluoride, or phosphoric acid. All of the examples of Lowrance employ a diluent to facilitate removal of water from the reaction vessel and force the esterification to substantial completion. It appears from Example 3 of Lowrance that yields are approximately 94% when a monocarboxylic acid is reacted with a phenolic compound.
While Lowrance's process appears to be an advance in the art, the Lowrance process has certain drawbacks, particularly the use of diluents for carrying out the reaction. Further, nonvicinal benzene dicarboxylic acids are harder to esterify than benzene monocarboxylic acids. This is unfortunate since the phenyl esters of nonvicinal dicarboxylic acids are potentially useful intermediates for the production of the so-called polyarylates, i.e., ester interchange reaction products of phenyl esters of nonvicinal benzene dicarboxylic acids and aromatic polyhydric alcohols such as Bisphenol A, etc. Accordingly, there is a need for additional catalyst systems for the production of esters of phenolic compounds.
The general object of this invention is to provide a new process for the production of phenolic esters of carboxylic acids. A more specific object of this invention is to provide a new process for the production of diphenyl esters of nonvicinal benzene dicarboxylic acids. Other objects appear hereinafter.
We have now found that the objects of this invention can be attained by reacting a phenolic compound with a carboxylic acid using a catalyst composition consisting essentially of an alkali metal compound and a boron compound wherein said alkali metal compound is at least one member selected from the group consisting of alkali metal salts of boric acid, alkali metal polyborates, alkali metal hydroxides, and alkali metal borohydrides and said boron compound is at least one member selected from the group consisting of boric acid, boric anhydride, alkali metal polyborates, alkali metal salts of boric acid and alkali metal borohydrides. Surprisingly, these catalysts are more effective than the borate-sulfuric acid complexes of Lowrance. Both the alkali metal component and boron component are essential. For example, manganese borate and zinc borate cannot be used in place of the alkali metal component and boron component.